Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED includes, in sequence, an anode, an EL medium, and a cathode. The EL medium disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface at the HTL. Tang et al. (Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
The luminance output and the operational lifetime of OLEDs are important device parameters, which are controlled by many factors. One of the factors is the driving current. Van Slyke et al. (Applied Physics Letters, 69, 2160 [1996]) indicated that the luminance generally measured in units of candela per square meter (cd/m2) is proportional to the current density passing through the device, and the lifetime of the device is inversely proportional to the current density.
Thus, there is unavoidable tradeoff between luminance and operational lifetime. Accomplishing both high luminance and long operational lifetime is therefore very advantageous and would enable a much wider range of applications for OLEDs.
A method to improve the luminance and efficiency of a multicolored or full color RGB OLED has been disclosed by Forrest et al. in U.S. Pat. No. 5,703,436. In the method of Forrest, an OLED device is fabricated by vertically stacking multiple, individually addressable OLED units, each emitting light of a different color. Electrodes are provided between OLED units (intra-electrodes) as a means of independently controlling each individual OLED unit. Specifically, this stacked OLED requires substantially transparent intra-electrode layers made of metallic or inorganic materials for electrical conductivity. It also requires bus lines for providing electrical power to control each individual OLED unit. In contrast to side-by-side construction of conventional RGB sub-pixels in an OLED display, Forrest et al. teach that the stacked orientation in this invention allows each color sub-pixel to be spread over a greater area, and thus, operate at a lower current density. However, the total current requirements for the device are not reduced, and the luminance efficiency in terms of candela per ampere (cd/A) is actually not improved. In U.S. Pat. No. 6,274,980, Burrows et al. propose that one can use the method of Forrest et al. to increase the luminance capability of an OLED device by stacking multiple OLED units, all of which emit the same color. While this enables higher luminance, the overall construction of the OLED device is complex, requiring transparent electrodes as well as separate power source for each of the stacked OLED units.
The use of metallic or inorganic intra-electrodes in the stacked OLED of Forrest et al. or Burrows et al. poses several problems. First, it requires complex wiring in order to address each OLED unit in the stack. Second, the electrode layers between the OLED units in the stack absorb light, and if made too thick, create optical losses that decrease overall light output efficiency. So-called transparent electrodes are known but still absorb significant levels of light and are difficult to fabricate over organic structures. In fact, Burrows et al. show by mathematical modeling the serious decrease in luminance efficiency caused by the expected optical losses due to the intra-electrode layers. Third, if the electrode is made too thin to improve optical properties, the sheet resistance is too high rendering high driving voltage for each individual OLED element and uneven luminance inside the device area.
In an alternative design, Jones et al. (U.S. Pat. No. 6,337,492) disclose a stacked OLED structure with a conductor layer between the individual OLED units without individually addressing each OLED unit in the stack. These conductor layers are basically equivalent to the intra-electrode layer of Forrest et al., except that they are not connected to a power supply. While this alleviates the complex wiring problems of U.S. Pat. No. 5,703,436 and U.S. Pat. No. 6,274,980, the device disclosed by Jones et al. suffers the same optical problems as noted above. The conductor layers are preferably 0.1 to 15 nm thick, and include allegedly transparent metal alloys, metal oxides, and other well known inorganic electrode materials commonly used in OLED devices, all of which have unwanted absorption and light-scattering effects. Jones et al. proposes that the device structure can be used to make OLED devices with higher luminance efficiency and higher operational stability, but offers no working examples. Neither does Jones et al. suggest how to make a useful device without a conductor layer.